Active Matrix Organic Light Emitting Diode (AMOLED) displays are hot in current flat panel display research. As compared with a liquid crystal display (LCD), an organic light emitting diode (OLED) has advantages such as low power consumption, low production costs, self-luminous, wide viewing angle, short response time and so on. At present, in the display field, such as mobile phones, PDAs, digital cameras and the like, the OLED display screens are taking places of traditional LCD display screens. Among others, pixel driving is core technical content for the AMOLED display, and has important research value.
Unlike thin film transistor-Liquid Crystal Displays (TFT-LCD), which use a stable voltage to control brightness, OLEDs are driven by current and require a constant current to control light emission. As shown in FIG. 1, a traditional AMOLED pixel driving circuit utilizes a 2T1C pixel driving circuit. The circuit consists of only one driving thin-film transistor T1, one switch thin-film transistor T2 and one storage capacitor C. When the scanning line gates (i.e. scans) a certain line, the scanning signal Vscan is a high level signal, the transistor T2 is turned on, and the data signal Vdata is written into the storage capacitor C. After the line is completely scanned, Vscan turns to be a low level signal, the transistor T2 is turned off, and the gate voltage stored in the storage capacitor C drives the transistor T1 so that the transistor T1 generates a current to drive the OLED and ensures that the OLED continuously emits light during one frame. The current to drive the thin film transistor T1 at a saturation state is represented as Ioled=K(Vgs−Vth)2, where K is a parameter related to process and design, Vgs is a gate-source voltage for driving the thin film transistor, and Vth is a threshold voltage for driving the thin film transistor. Once the size and process for the transistor are determined, the parameter K is determined. FIG. 2 shows an operation timing chart of the pixel driving circuit as shown in FIG. 1, where the timing relationship between the scanning signal supplied from the scanning line and the data signal supplied from the data line are shown.
An AMOLED is driven by a current generated in a saturated state of the driven thin film transistors (DTFT), so that it is capable of emitting light. Difference of threshold voltages may exist for driving thin film transistors at different locations, due to process non-uniformity, regardless of a low-temperature polysilicon (LTPS) process or an oxide process. This difference is fatal for the uniformity of the current-driven devices since different threshold voltages generate different driving currents when the same drive voltages are applied, resulting in inconsistency of the currents flowing through the OLED leading to non-uniform display brightness, and thus affecting displaying effect of the display panel.
Therefore, there is a need for a method which can improve the uniformity of the driving current of the driving transistor and thereby improve the display quality.